The electrically-variable transmission (EVT) has been proposed for vehicles to improve fuel economy and reduce emissions. The EVT splits power between an input shaft and an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path may include clutches and additional gears. The electrical power path may employ two electrical power units, each of which may operate as a motor or as a generator (“motor/generator”). With an electric storage battery, the EVT can be incorporated into a propulsion system for a hybrid electric vehicle.
The hybrid vehicle or hybrid propulsion system uses an electrical power source, such as a battery, as well as an engine power source. The battery is connected with the electrical drive units through an electronic control unit (ECU), which distributes the electrical power as required. The ECU also has connections with the engine and vehicle to determine operating characteristics, or operating demand, so that the electrical power units are operated properly as either a motor or a generator. When operating as a generator, an electrical power unit accepts power from either the vehicle or the engine and stores the power in the battery, or provides that power to operate another electrical device or another electrical power unit on the vehicle or on the transmission.
One of the benefits of having an electrically-variable transmission incorporating more than one mode of operation is that each mode of operation will generally incorporate at least one mechanical point at which no electrical energy is present in either motor/generator, thereby reducing losses.
A power transmission in an electromechanical transmission is described in commonly assigned United States Provisional Application entitled “Electrically Variable Transmission with Selective Fixed Ratio Operation,” U.S. Ser. No. 60/590,427, Holmes et al., filed Jul. 22, 2004 and is hereby incorporated by reference in its entirety.
One current issue with gasoline/electric vehicles is their ability to effectively operate in reverse. Previous gasoline/electric vehicles generally operate in reverse in two ways. One method of operating in reverse is to use the electric motor to propel the vehicle in reverse. Since the engine attempts to propel the vehicle forward, the electric motor must overpower the running engine. Such counteraction between the engine and the electric motor causes large losses because only a portion of electric motor power is used to reverse the vehicle. A second method of operating in reverse uses the electric motor to drive the vehicle in reverse while the engine is not running. This method, however, does not permit the vehicle to generate energy while reversing. Thus, it only permits the vehicle to operate in reverse until the battery is depleted or discharged. Once the battery has exhausted all of its energy and is fully discharged the vehicle will not be capable of reversing any further.